You will need to use the export utility from the lowest version of the RDBMS.
In your case you will need to use the 8i one.
Before using EXP utility of Oracle8i to export Oracle 9i database, you may have to execute catexp8.sql (must be in
oracle9i_home/rdbms/admin/) on Oracle 9i database. This sql script creates lower version views required for oracle8i exp utility.
Then as Jim said, use Oracle 8i exp utility to export Oracle9i databaase and import using 8i IMP utility...